You could also just use a substrate that buffers the water. ADA aquasoil, Azoo plant grower, and Up Aqua soil are a few options.

ETA: You could also just use your tap as is. I don't know about the cardinals, but I have cories, otos, and other soft water fish that are fine and healthy in my tap which sounds very similar to yours. It reads 7.4 or 7.6 depending on which API test I use and has a gh of 12.

I can't even imagine two Bostons! This little guy keeps me busy enough. Thanks for the link, that was the basic plan I was going to try to follow. Seems to be the easiest for a busy schedule.